Low-salt process for the preparation of a polysulfide

ABSTRACT

wherein R1 and R2 can be the same or different and are selected from alkane chains containing 2-10 carbon atoms, X is a halogen atom, and n, m, and p are integers that can be the same or different and have a value in the range 1-6, with either (i) sodium polysulfide or (ii) a combination of sodium hydrosulfide and sulfur.

The present invention relates to a process for the preparation of a polysulfide.

Polysulfides are a class of polymers (in the following polysulfides) with alternating chains of several sulfur atoms and hydrocarbons. The general formula for the repeating unit is —[R—S_(x)]_(n)—, wherein x indicates the number of sulfur atoms, n indicates the number of repeating units, and R indicates the organic backbone of the polymer. Cured polysulfide polymers are resistant to ageing and weathering, highly elastic from −40 to +120° C., and they offer an outstanding chemical resistance, especially against oil and fuel. Because of their properties, these materials find use as base polymers for sealants applied to fill the joints in pavement, insulation glass units, and aircraft structures.

Polysulfides are conventionally synthesized by reactions between organic dihalides and alkali metal salts of polysulfide anions. Conventional types of polysulfides include solid and liquid polymers.

Solid polymers have a molecular weight of about 10⁵ g/mol and are prepared from dihaloalkanes (such as 1,2-dichloroethane), alone or in admixture with a bis(2-chloroalkyl)formal, for example bis(2-chloroethyl)formal, and optionally a branching agent such as 1,2,3-trichloropropane.

Liquid polysulfides have a molecular weight of about 10² to 10³ g/mol and are generally prepared from a bis(2-chloroalkyl)formal and optionally small amounts of a branching agent like 1,2,3-trichloropropane. The resulting polysulfide is then split into chains of the required lengths by reduction of the disulfide linkages.

A disadvantage of this process is that it does not allow much control over the polarity of the resulting polysulfide.

The polarity of the polysulfide affects its compatibility with surfaces. Polysulfides are often used as sealants for double glazing and in aircrafts. Hence, good compatibility with relatively polar surfaces like glass and metals such as aluminium or steel is required for these applications. The polarity is improved with the introduction of more oxygen relative to sulfur atoms. In addition, the flexibility and elasticity of the polymer at low temperatures and the compatibility of the polymer with plasticizers is improved with higher oxygen contents. On the other hand, the chemical resistance against oil and jet fuel improves with a higher content of sulfur relative to oxygen atoms. For aircraft applications, for instance, this leads to conflicting requirements for the sulfur/oxygen ratio of the polymer.

It would therefore be desirable to provide a process that would allow control over the oxygen and sulfur content of the resulting polymer and the possibility to easily adapt this ratio depending on the particular requirements of the product.

Preparing liquid polymers by splitting the chains of solid polymers derived from either dichloroalkane or a combination of dichloroalkane and bis(2-chloroalkyl)formal would not solve these problems because it would lead to liquid polysulfides having a relatively high sulfur content, a relatively low oxygen content and, thus, a relatively low polarity, without suitable means to adapt these properties.

Applicant's unpublished patent application PCT/EP2014/062306 (Process for the preparation of a polysulfide) discloses a process to prepare polysulfides by reacting a bis(2-dihaloalkyl)formal and a dihaloalkane with sodium polysulfide n the presence of a pre-polymer according to structure (I) X—(R2—O)n—CH2—O—(R1—O)m—CH2—(O—R2)p—X wherein R1 and R2 can be the same or different and are selected from alkane chains containing 2-10 carbon atoms, X is a halogen atom, and n, m, and p are integers that can be the same or different and have a value in the range 1-6.

A further disadvantage of the known processes is the formation of large quantities of salt because of the high chlorine content of the used halides. This salt has to be disposed of which has a negative environmental impact and needs an additional economic expenditure.

It is an object of the present invention to provide a liquid polysulfide polymer with good chemical resistance and compatibility with plasticizers and polar surfaces wherein the process allows control over the oxygen and sulfur content of the resulting polymer and the possibility to easily adapt this ratio depending on the particular requirements of the product In addition the process should allow for the preparation of polysulfides with an improved salt balance compared to the known processes while the prepared polysulfide should have comparable application properties like the conventional polysulfides.

This problem has now been solved by the low-salt process according to the present invention for the preparation of a polysulfide comprising the step of reacting in the absence of a dihaloalkane a mixture of bis(2-dichloroalkyl)formal in the presence of the pre-polymer according to structure (I) X—(R²—O)_(n)—CH₂—O—(R¹—O)_(m)—CH₂—(O—R²)_(p)—X (I), wherein R¹ and R² can be the same or different and are selected from alkane chains containing 2-10 carbon atoms, preferably 2-6, and most preferably 2-4 carbon atoms, X is a halogen atom, and in, m, and p are integers that can be the same or different and have a value in the range 1-6, preferably 1-4, with either (i) sodium polysulfide or (ii) a combination of sodium hydrosulfide and sulfur.

It is an advantage of the present process that by-product formation, thus salt formation, is reduced compared to the known processes which has an improved environmental impact as less by-product needs to be disposed of. The reduced amount of salt results from the lower halogen content of the reaction mixture.

Preferably, X is a halogen atom selected from CI, Br, and I, more preferably Cl, Preferably, R¹ is —CH₂—CH₂—.

The preferred nature of R² is —CH₂—CH₂—, —CH₂—CH₂—CH₂—, or —CH₂—CH₂—CH₂—CH₂—.

The dihaloalkane which is to be absent from the process has the formula X—R—Y, wherein X and Y are both halogen atoms that may be the same or different, and R is an alkane chain.

The pre-polymer according to structure (I) is obtainable by reacting a polyol with (para)formaldehyde and a halo-alcohol in the presence of an acid catalyst.

Suitable polyols include monoethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, monopropylene glycol, dipropylene glycol, tripropylene glycol, 1,4-butanediol, and mixtures thereof.

Suitable halo-alcohols include chloroalcohols, bromoalcohols, and iodoalcohols, whereby chloroalcohols are preferred. Examples of suitable chloroalcohols are ethylene chlorohydrin (ECH), propylene chlorohydrins, butylene chlorohydrins, pentylene chlorohydrins, and hexylene chlorohydrins. ECH is the most preferred chloroalcohol.

Suitable acid catalysts are HBr, HCl, H₂SO₄, H₃PO₄, p-toluene sulfonic acid, sulfonic acid, ferric chloride, and cation exchange resins, such as Amberlyst® 15, 31, 35,36,39,119, 131, Lewatite® K1131, K2431, K 2621, and Nafion® SAC-13.

The amount of acid catalyst is preferably in the range of from 0.1 to 10 wt %, based on the weight of entire reaction mixture.

In this specification, the term “(para)formaldehyde” includes formaldehyde (i.e. CH₂O) and the condensation products of formaldehyde having the formula (CH₂O)_(n) that are conventionally referred to as paraformaldehyde. The value of n in this formula is generally in the range 8-100. In the present invention, the use of paraformaldehyde is preferred over formaldehyde.

In the formation of the pre-polymer, the molar ratio of (para)formaldehyde (calculated as CH₂O) to OH-functionalities of the polyol is preferably in the range 0.8:1-1.5:1, more preferably 0.9:1-1.3:1, and even most preferably 0.9:1-1.2:1.

The molar ratio of halo-alcohol to OH-functionalities of the polyol is preferably in the range 0.9:1-1.5:1, more preferably 0.9:1-1.4:1 and most preferably 1:1-1.2:1.

The molar ratio of (para)formaldehyde (calculated as CH₂O) to halo-alcohol is preferably in the range 0.8:1-1.5:1, more preferably 0.9:1-1.3:1, and most preferably 0.9:1-1.2:1.

The reaction towards the pre-polymer is preferably performed by heating the reaction mixture to a temperature in the range 45-80° C., more preferably 50-75° C., and most preferably 55-65° C. This heating is preferably conducted for 10 minutes to 2 hours, more preferably 20 minutes to 1.5 hours and most preferably 30-60 minutes.

This heating step is preferably followed by two azeotropic distillation steps in order to remove reaction water and any excess of halo-alcohol, thereby shifting the equilibrium towards the pre-polymer.

Suitable bis(2-dihaloalkyl)formals for use in the process of the present invention are bis(2-dichloroalkyl)formals, bis(2-dibromoalkyl)formals, and bis(2-diiodoalkyl)formals. The most preferred bis(2-dihaloalkyl)formal is bis(2-dichloroethyl)formal: Cl—C₂H₄—O—CH₂—O—C₂H₄—Cl.

Sodium polysulfide has the formula Na2S_(x), wherein x is in the range 2-5, preferably in the range 2-3, and most preferably in the range 2.2-2.5. The molar ratio of sodium polysulfide (calculated as Na₂S_(x)), relative to bis(2-dihaloalkyl)formal, is preferably in the range 0.8-1.4, more preferably 0.9-1.3, and most preferably 1.0-1.2. Instead of sodium polysulfide, also a mixture of sodium hydrosulfide (NaHS) and sulfur (S) can be used. Preferably the mixture of NaHS and sulfur is an aqueous solution. This has the additional advantage that a splitting step is generally not required.

The weight ratio bis(2-dihaloalkyl)formal to pre-polymer to be used in the process according to the present invention is preferably in the range 90:10 to 10:90, more preferably in the range 70:30 to 30:70, even more preferably in the range 40:60 to 60:40, and most preferably in the range 45:55 to 55:45.

Optionally a branching agent can be present in the process of the invention. The branching agent serves to form a three dimensional crosslinked structure after curing of the polysulfide and, consequently, a reinforced hardness with good elastic properties of the cured polymer. The branching agent preferably is a trihalide, more preferably 1,2,3-trichloropropane. The branching agent is preferably present in the mixture in an amount of 0.5 to 2 wt %, relative to the weight of bis(2-dihaloalkyl)formal. The branching agent is preferably not a branching agent selected from the group consisting of di-aldehydes and their corresponding acetals and hemiacetals. This means that the process according to the invention is preferably performed in the absence of a branching agent selected from the group consisting of di-aldehydes and their corresponding acetals and hemiacetals.

In one embodiment, the process of the invention is performed by first preparing a mixture comprising the bis(2-dihaloalkyl)formal, the pre-polymer, and optionally the branching agent, and adding this mixture to an aqueous solution of sodium polysulfide and alkali metal hydroxide whereby the process is performed in the absence of a dihaloalkane. Optionally, a dispersing agent, such as magnesium hydroxide, and/or a wetting agent (e.g. sodium butylnaphthalenesulfonate) may be present in the solution.

The mixture is preferably added slowly, e.g. dropwise, to the solution. The temperature of the solution is preferably in the range 60 to 100° C., more preferably from 80 to 95° C. and most preferably from 85 to 90° C.

In a second embodiment, the process of the invention is performed by first preparing a mixture comprising bis(2-dihaloalkyl)formal and the pre-polymer (I), and optionally the branching agent, and adding this mixture to an aqueous solution of NaHS and sulfur. This embodiment has the additional advantage of a simple process design. The bis(2-dihaloalkyl)formal-containing mixture is preferably added slowly, e.g. dropwise, to the NaHS and sulfur solution. The temperature of the NaHS and sulfur solution is preferably in the range 60 to 100° C., more preferably from 80 to 100° C. and most preferably from 90 to 100° C. Preferably, a phase transfer catalyst (PTC), such as a quaternary ammonium compound, is added to the mixture. A PTC according to the invention is a catalyst which facilitates the migration of a reactant from a first phase into a second phase, wherein the reaction occurs in the second phase. The polymer obtained in this embodiment is a liquid polysulfide.

In a further embodiment, the process according to the invention is performed by adding the reactants to one reactor, preferably sequentially.

As a subsequent step, the resulting reaction mixture is preferably treated with a desulfurization agent (e.g. sodium hydroxide and sodium hydrogen sulfide) to eliminate any labile sulfur atoms. This desulfurization step can be conducted at a preferred temperature of 80-110° C., more preferably 85-105° C., and most preferably 90-100° C. The reaction time is preferably 1-4 hours, more preferably 1-3 hours, and most preferably 1-2 hours.

In case of reacting the bis(2-dihaloalkyl)formal in the presence of the pre-polymer (I) with sodium polysulfide according to the invention, the macromolecules in the resulting polysulfide polymer need to be reduced to the required chain length by reductive splitting of the disulfide bonds to obtain a liquid polysulfide. The most common reduction agents are sodium dithionite (Na₂S₂O₄) and a combination of NaSH and Na₂SO₃. The amount of reduction agent to be used depends on the desired molecular weight, as commonly known in the art.

The preferred reduction agent in the process according to the invention is sodium dithionite. Reductive splitting using sodium dithionite is preferably performed for 20-40 minutes. The temperature preferably ranges from 80 to 110° C., more preferably from 85 to 105° C. and most preferably from 90 to 100° C. The reaction time is preferably 1-4 hours, more preferably 1-3 hours, and most preferably 1-2 hours.

If desired, the splitted disulfide bonds can then be converted into reactive terminal thiol groups by acidification at pH 4-5. Acetic acid is preferably used as acidifier.

As a last step, the polysulfide can be washed and dewatered under reduced pressure.

The polysulfide resulting from the process of the present invention has various applications, including the use as binder in sealants, adhesives, and coating compositions, in isocyanate cure, in epoxy-resin cure, and in acrylate resin cure. 

1. Process for the preparation of a polysulfide comprising the step of reacting a bis(2-dihaloalkyl)formal with either (i) sodium polysulfide or (ii) a combination of sodium hydrosulfide and sulfur, the reaction being performed in the absence of a dihaloalkane and in the presence of a pre-polymer (I) according to structure (I) X—(R²—O)_(n)—CH₂—O—(R¹—O)_(m)—CH₂—(O—R²)_(p)—X  (I), wherein R¹ and R² can be the same or different and are selected from alkane chains containing 2-10 carbon atoms, X is a halogen atom, and n, m, and p are integers that can be the same or different and have a value in the range 1-6.
 2. Process according to claim 1, wherein the bis(2-dihaloalkyl)formal is bis(2-dichloroalkyl)formal.
 3. Process according to claim 1 wherein the X is a halogen selected from Cl, Br, and I.
 4. Process according to claim 3 wherein the X is Cl.
 5. Process according to claim 1, wherein the R¹ of the pre-polymer (I) is —CH₂CH₂—.
 6. Process according to claim 1, wherein the R² of the pre-polymer (I) is —CH₂—CH₂—, —CH₂—CH₂—CH₂—, or —CH₂—CH₂—CH₂—CH₂—.
 7. Process according to claim 1, wherein the product resulting from the reaction of the bis(2-dihaloalkyl)formal with sodium polysulfide in the presence of the pre-polymer (I) is treated with a reduction agent to obtain a liquid polysulfide.
 8. Process according to claim 1, wherein the molar ratio of sodium polysulfide (calculated as Na₂S_(x)) relative to the bis(2-dihaloalkyl)formal is in the range 0.8-1.4.
 9. Process according to claim 1, wherein a mixture comprising bis(2-dihaloalkyl)formal and the pre-polymer (I) is added to an aqueous solution of sodium hydrosulfide and sulfur.
 10. Process according to claim 9, wherein the aqueous solution has a temperature in the range of 60 to 100° C.
 11. Process according to claim 1, wherein the weight ratio of the bis(2-dihaloalkyl)formal to the pre-polymer (I) is in the range of 90:10 to 10:90.
 12. Product obtained by the process according to claim
 1. 13. A composition comprising a product obtained by the process according to claim 1, said composition selected from sealants, adhesives and coating compositions. 